1. Field of the Invention
The present invention relates to a capacitive micromachined ultrasonic transducer.
2. Description of the Related Art
Ultrasonic diagnosis methods in which ultrasonic waves are transmitted inside a body cavity are widely employed and the condition of the body cavity is visualized on the basis of the echo signals. An ultrasonic endoscope is one device used for ultrasonic diagnosis. In an ultrasonic endoscope, an ultrasonic transducer is provided to the distal end of an insertion unit, which is inserted into body cavities. This transducer converts electric signals into ultrasonic waves for transmitting them inside body cavities, and also receives reflected ultrasonic waves in the body cavities in order to convert them into electric signals.
Some ultrasonic transducers have a configuration including, for example, a circular and concave ultrasonic reflection plane and a planar back plane, i.e., a plano-concave configuration (see, for example, Japanese Patent Application Publication No. 2003-299195 and Japanese Patent No. 3478874). In the concave ultrasonic reflection plate, the central spot has the minimum thickness, and the closer to the periphery a spot is, the greater the thickness of the spot is.
A piezoelectric element is included in an ultrasonic transducer, which converts ultrasonic waves and electric signals into each other. The piezoelectric element generates ultrasonic waves at different frequencies in accordance with the thickness of respective portions in the element itself. Specifically, ultrasonic waves at low frequencies are generated at portions having a great thickness, and ultrasonic waves at high frequencies are generated at portions having a small thickness because ultrasonic wave frequency is in inverse proportion to the thickness of the piezoelectric element.
Therefore, in an ultrasonic transducer having a circular and concave ultrasonic reflection plate i.e., in an ultrasonic transducer having the plano-concave configuration, the ultrasonic wave at the highest frequency is generated at the central spot, and the closer to the periphery a spot is, the lower the frequency of the wave transmitted from the spot is.
The purpose of generating ultrasonic waves at different frequencies as above is to obtain images at different resolutions based on the different frequencies. By using ultrasonic waves at high frequencies, image information of the surface and around it can be obtained at a high resolution. However, ultrasonic waves at high frequencies are subjected to attenuation at deeper portions. When ultrasonic waves at low frequencies are used, the resolution of image information is lower than that of the image information obtained by using the ultrasonic waves at high frequencies. However, the ultrasonic waves at low frequencies are less subjected to the attenuation, and can therefore be used for observation of deeper portions. Accordingly, by synthesizing the ultrasonic waves at high frequencies and the ultrasonic waves at low frequencies, it is possible to obtain ultrasonic images of shallower to deeper portions at relatively high resolution.
Additionally, for ultrasonic image processing methods and ultrasonic image processing apparatuses using parametric sound source methods, an ultrasonic image processing apparatus using the parametric array has been disclosed in recent years, in which the attenuation of echo signals can be reduced (see, for example, Japanese Patent Application Publication No. 8-80300).
Japanese Patent Application Publication No. 8-80300 discloses the following method. By transmitting from an ultrasonic probe to a sample of an amplitude-modulated wave whose center frequency is amplitude-modulated or an ultrasonic wave having two frequency components, an echo having a frequency component of a difference that is based on the nonlinearity of tissues is generated in the sample. Because the echo having the frequency component of the difference is lower than the fundamental frequency, the attenuation of the signal strength while the wave is being transmitted through the sample becomes much smaller.
The parametric array used herein is a sound source whose beam pattern is sharper than that of a wave at a frequency that is the same as that of a difference tone between waves at different frequencies. An effect that results from the acoustic characteristic (parametric characteristic) achieved by this parametric array is called a parametric effect.
Recently, capacitive micromachined ultrasonic transducers (referred to as c-MUT hereinafter) have been attracting interest. The capacitive micromachined ultrasonic transducer is one of several devices that are categorized into MEMS (Micro Electro-Mechanical Systems).
A MEMS device is a device that is formed as a microstructure on a substrate such as a silicon substrate, a glass substrate, or the like. In a MEMS device, driven bodies for outputting mechanical forces, driving mechanisms for driving the driven bodies, semiconductor integrated circuits for controlling the driving mechanisms, and the like are electrically and mechanically connected. A MEMS device is mainly characterized by the configuration in which the driven bodies that are configured as mechanisms are incorporated into the device. The driven bodies are electrically driven by using the Coulomb attraction between electrodes.
A capacitive micromachined ultrasonic transducer (c-MUT) is a device in which two planar electrodes are arranged such that they face each other, having a cavity between the electrodes. The capacitive micromachined ultrasonic transducer transmits ultrasonic waves when it receives AC signals superposed on a DC bias by having a layer (membrane) including one of the above electrodes oscillate harmonically to the AC signals received.